Evaluation of mean glycemic control in diabetic patients is possible with the measure of glycosylated hemoglobin (HbA1c). Recently, new recommendations were made giving this test a diagnostic value for diabetes. HbA1c is the best prediction parameter of complications linked to diabetes; its measure is relatively accurate and simple. Nevertheless, the threshold value is arbitrary and a standardized method of dosage still remains limited. The aim of this article is to analyze the implementation of this dosage, its advantages and disadvantages as a diagnostic tool in comparison with plasma glycaemia, and current data concerning its sensibility, specificity and clinical implication.